Tripartite ER-mitochondria-lipid droplets contact sites control adipocyte metabolic flexibility.

Adipocyte dysfunction is a major driver of obesity-associated cardiometabolic disease, underscoring the need to understand how lipid storage and mobilization are regulated and disrupted. The ER-anchored protein Seipin governs lipid droplet (LD) biogenesis and ER-LD and ER-mitochondria (MAM) contacts, and its loss impairs calcium transfer and causes lipodystrophy. Here, we investigated whether Seipin coordinates MAM and ER-LD remodeling during adipocyte lipid handling. In subcutaneous adipose tissue from inducible Seipin-knockout mice, electron microscopy and proximity ligation assays revealed that feeding reduces MAMs while increasing ER-LD and mitochondria-LD contacts, a remodeling abolished by Seipin deficiency. Lipid loading elevated tripartite MAM-LD contacts in controls but not knockouts. Fluorescence recovery after photobleaching showed that impaired triglyceride transfer to LDs in Seipin-deficient cells was rescued by the MAM-LD-stabilizing peptide 'Linker-ER-Mi', in a calcium-dependent manner. During adipogenesis and lipid loading, MAM-LD contacts increased, whereas MAM-cytosolic mitochondria contacts declined; however, obesity blunted this remodeling. Furthermore, disrupting membrane contact sites impaired lipid flux, lipolysis, and insulin signaling. Taken together, these findings identify MAM-LD as regulators of adipocyte metabolic flexibility.